Cellulosic Ethanol on the Cheap

Microbe breweries: A new technique employs engineered superbugs to turn cellulose directly into ethanol from sources such as (pictured from left to right) corn stover, wood chips and paper sludge.

The process of making ethanol from cellulosic sources such as wood chips and paper pulp is somewhat like following a complicated French recipe: it takes many costly ingredients and multiple pots, each with its own settings and instructions, to concoct the final product, and the entire enterprise is expensive and somewhat inefficient. Now Mascoma, a cellulosic biofuels company based in Lebanon, NH, reports significant advances in its goal of simplifying the cellulosic ethanol process by skipping the use of costly enzymes, which could potentially reduce cellulosic ethanol’s production costs by 20 to 30 percent.

Mascoma’s strategy, called consolidated bioprocessing, aims to combine the multiple steps of ethanol production into one, using genetically engineered superbugs that perform the multiple steps involved in making cellulosic ethanol. The company reports a series of advances that it says brings it “substantially closer to commercialization.” Mascoma announced the results recently at the 31st Symposium on Biotechnology for Fuels and Chemicals, in San Francisco.

Existing technology to produce ethanol from cellulosic sources involves a multistep process: plant material such as paper pulp and switchgrass are first pretreated, to separate cellulose from the rest of the plant matter. Cellulose is then mixed with enzymes that break it down into sugars. Yeast then takes over to ferment the sugars into ethanol.

As a less costly alternative, Mascoma researchers are engineering microbes to combine the last two steps of the process: breaking down cellulose, and converting sugars into ethanol. They say that if they can get microorganisms to make ethanol at sufficiently high rates, they can reduce the amount of expensive enzymes needed to break down cellulose, which can normally take up half of ethanol’s production costs.

The company is exploring three potential organisms for ethanol production: two types of bacteria, and one yeast strain. C. thermocellum and T. saccharolyticum are thermophilic bacteria, able to withstand high temperatures such as those experienced in reactors. Researchers have been interested in both bacterial strains for years due to their natural ability to both convert cellulose into sugar and ferment sugar into ethanol.

However, these strains produce very low levels of ethanol. The limiting factor is its by-products: both bacteria break down cellulose into glucose and other sugars such as xylose. The bacteria can then ferment glucose into ethanol, but remaining sugars like xylose cannot be fermented. What’s more, ethanol yield is low because bacteria produce other organic acid by-products in the fermentation process, such as acetate and lactate. Scientists have also found that these bacteria are inhibited and stop growing in the presence of high levels of ethanol.